Thermoelectric device with graphite elements



June 24, 1969 A. G. F. DINGWALL THERMOELECTRIC DEVICE WITH GRAPHITE ELEMENTS Filed Oct. 23, 1965 IN VENTOR. 13m?! IA'fi/MMM United States Patent Q 3,451,858 THERMOELECTRIC DEVICE WITH GRAPHITE ELEMENTS Andrew G. F. Dingwall, Cedar Grove, N.J., assig nor to Radio Corporation of America, a corporation of Delaware Filed Oct. 23, 1965, Ser. No. 502,946

Int. Cl. H01v 1/30 U.S. Cl. 136205 1 Claim This invention relates to thermoelectric devices, and particularly to thermal pressure contacts or joints used in certain types of thermoelectric devices.

Thermoelectric devices operating in accordance with the Seebeck effect comprise a pair of opposite conductivity type thermoelements each having a hot and a cold end. Heat is applied to the hot ends of the thermoelements and the cold ends are cooled.

In one type of thermoelectric device, the hot ends of the thermoelements are engaged with a heat source, such as a tube through which a heated fluid is circulated, and the cold ends of the thermoelements are engaged with a heat sink, such as a radiator. In each case, heat passes to and from the ends of the thermoelements by conduction. It is known that for highest efficiency of such thermoelectric devices, the contacts or joints between the ends of the thermoelements and the heat source and sink should have the highest possible thermal conductivities.

The thermoelements are rigid members, and to prevent cracking thereof due to thermal expansion stresses caused by the difference in temperature between the hot and cold ends of the thermoelements, at least one end of each thermoelement is nonfixedly engaged with its contacting heat source or sink. Preferably, a pressure sliding contact is used. Thermal differential expansion stresses in a direction perpendicular to the contacting surfaces in the contact or joint are taken up by a resilient pressure means holding the contacts together, and the thermal differential expansion stresses in a direction parallel to the contacting surfaces are taken up or relieved by sliding movement of the surfaces relative to one another.

A problem in the past has been the provision of pressure contacts having both high thermal conductivity and good sliding properties. Heretofore, high thermal conductivity across the contacts required the use of relatively high pressures to provide intimate surface engagement between the contacting members. In most instances, however, the desired amount of pressure is greater than the pressure that the thermoelements can withstand. Further, the use of pressure in the past was incompatible with sliding contacts to relieve stress. With pressure, and especially in high temperature vacuum operation, prior known contacts tend to seize and bind together. Failure of the contacts to slide relative to one another generally results in cracking of the thermoelements.

An object of this invention is to provide novel and improved thermal pressure contacts for thermoelectric devices.

A further object of this invention is to provide novel and improved thermal pressure contacts for thermoelectric devices operable at high temperatures in vacuum, the thermal conductance across the contacts being high, and the surfaces of the contacts being readily slidable with respect to one another.

For achieving these objects, one or more of the surfaces on the pressure joint or contact is made of graphite. Various ways in which the graphite may be provided, and various advantages arising from the use of the graphite are described hereinafter.

In the drawings:

FIG. 1 is a side elevation of a thermoelectric device; and

FIG. 2 is a fragmentary elevation view of a modification of the device shown in FIG. 1.

With reference to FIG. 1, a thermoelectric generator 4 is shown mounted between a heat source 6 and a heat sink 8. The heat source 6 comprises, for example, a container 10 containing a radioisotope material such as strontium 90. The heat sink 8 comprises a radiator plate made from a high thermal conductivity material such as copper.

The thermoelectric generator 4, which is only one of numerous known thermoelectric generators with which this invention has utility, comprises N-type and P-type semiconductor thermoelements N and P, respectively, each of the elements comprising, for example, a silicongermanium alloy. These elements may be either polycrystalline or mono-crystalline. The thermoelement P is heavily doped with an electron acceptor element such as boron, aluminum, or gallium from Group IIIA of the Chemical Periodic Table, and the thermoelement N is heavily doped with an electron donor element such as phosphorous or arsenic from Group V-A of the Chemical Periodic Table.

A hot strap 11 extends between and is bonded to what is to become the hot ends of the thermoelements N and P. The hot strap 11 is formed from two shoes 12 and 14. The shoe 12 is made of an N-type silicon alloy and is joined to the hot end of the thermoelement N. The shoe 14 is made of a P-type silicon alloy and is joined to the hot end of the thermoelement P. The shoes 12 and 14 may be rectangular blocks substantially the same size and having abutting faces 15 which are bonded to one another. The surfaces of the shoes 12 and 14 in engagement with the heat source 6 are of sufficient size to receive an adequate quantity of heat from the heat source for efficient operation of the thermoelectric generator.

The silicon alloy shoes 12 and 14 may be bonded to each other and to the thermoelements N and P by means of hot pressed diffusion bonds. The bonds are nonrectifying and of low electrical resistance. The bond between the shoes 12 and 14 may be obtained by first applying a layer of material such as chromium, cobalt, iron, manganese, nickel, niobium, rhenium, rhodium, tantalum, titanium, zirconium, tungsten, or molybdenum between the shoes 12 and 14. A diffusion bond is formed by heating the shoes while the shoes are pressed together. The temperature to which the shoes are heated may be in the order of about of the melting point (in the centigrade scale), but below the melting point, of any of the shoe materials or eutectics that may be formed between the contacting materials. For example, where the intermediate metallic layer is of chromium or titanium, the temperature at which the bonding process is carried out is between 1050 C. and 1200 C., and the pressure may be between about 200 and 500 p.s.i. This operation is preferably carried out in a vacuum or in a neutral ambient, such as argon, for example. The aforementioned heat and pressure may be applied for a period ranging from one half minute to one hour until a solid diffusion between the metal intermediate layer and the abutting shoes has taken place.

The bond between each shoe 12 and 14' and its respective thermoelement N or P may be obtained in the same manner but without the use of an intermediate metal layer and with pressures in the order of 5-10 p.s.i.

A pair of metal shoes 18 and 20', preferably of tungsten, are fixed to what is to become the cold ends of the thermoelements N and P. Any suitable known bonding techniques may be used, such as brazing the tungsten shoes to the cold ends of the thermoelements with copper or a noble metal or an alloy of noble metals. Alternately, the tungsten shoes may be diffusion bonded to the thermoelements N and P by the application of heat and pressure in the same manner and at the same time the shoes 12 and 14 are bonded to the thermoelements. Tungsten is preferably used because its coefficient of thermal expansion is relatively closely matched to the coefficient of thermal expansion of the silicon-germanium alloy thermoelements N and P.

The hot strap 11 of the thermoelectric generator 4 is metallurgically bonded to container 10. Although not shown, a plurality of thermoelectric generators are normally bonded to the container 10 heat source. To prevent shorting of the various thermoelectric generators, an insulating plate 22 is preferably disposed between the hot strap 11 and the container 10. The container 10 may be made of stainless steel, and the insulating plate 22 is preferably made of a relatively high thermal conductivity ceramic material such as alumina. Bonds between the hot strap 11 and the insulating plate 22 and between the plate 22 and the container 10 may be made in known manner. For example, the top and bottom surfaces of the ceramic plate 22, but not the side surfaces, are molybdenum metalized using known methods. A wafer of copper, between 30 and 40 mils thick, is disposed between the insulating plate 22 and the container 10 and brazed to these members with a nickel-gold braze, commercially known as Nioro. A wafer of graphite, between 30 and 40 mils thick, is disposed between the insulating plate 22 and the silicon alloy hot strap 11 and is brazed to both these members using a nickel-titanium braze. The aforementioned brazes may be performed simultaneously by assembling the parts in a suitable jig and heating the assemblage, preferably in vacuum or in an inert atmosphere, at a temperature of from 1050 C. to 1200" C., with pressures in the order of 5 to 50 p.s.i., for a time between one and five minutes.

The purpose of the copper and graphite wafers are to compensate or relieve thermal expansion stresses caused by the differential thermal expansions of the various materials used in the bond of the thermoelectric generator 4 to the container 10.

Bonded to the cold end of each of the thermoelements N and P, that is, to the shoes 18 and 20, is a treated graphite wafer 24. In one embodiment, the wafers are 20 mils thick. Preferably, a small amount of silicon is fired onto a low electrical resistance graphite such as grade 886-8 graphite, manufactured by Speer Carbon Company. Grade 886-8 graphite has a high thermal conductivity parallel to its grain, in the order of 1.9 watts/ cm. C. at room temperature, has a modulus of elasticity of around 1X10 p.s.i., and has a coefficient of thermal expansion across its grain of about 4.2 10 C. The tungsten cold shoes 18 and 20 have a coeflicient of thermal expansion of about 5.0 10 C., hence the thermal expansion characteristics of the shoes 18 and the graphite wafers 24 are relatively closely matched.

Graphite material comes in a large number of grades having different physical characteristics. The selection of the grade of graphite is therefore dependent upon the particular thermoelectric generator used. Preferably, a graphite grade having a high thermal conductivity and a coeificient of thermal expansion reasonably matching (to within about :t1 l0 C.) the coefiicient of thermal expansion of the member to which the graphite is bonded is selected.

Several methods for bonding materials to graphite are known, see for example The Bonding of Graphite by Donnelly and Slaughter, in High Frequency Heating Review, volume 1, number 12, 1-5 (1962). A preferred method utilizes a nickel-gold or nickel-titanium braze. The member to be bonded to the graphite is nickel plated and a gold or titanium metal shim is placed between the graphite member and the nickel plated member. The braze is then made by heating the assembled bodies to a temperature suflicient to form the nickel-gold or nickeltitanium eutectic. ln one embodiment, a 0.1 mil nickel plated tungsten cold shoe 18 and 20 is brazed to a 20 mil thick graphite wafer using a one half mil thick titanium shim disposed therebetween. The assembled bodies are heated to a temperature between 1050 and 1200 C. for three minutes in a one tenth atmosphere of helium using a pressure sufiicient to maintain the bodies in contact during the brazing operation.

The graphite wafers 24 shown in FIG. 1 are not bonded to the heat sink 8. To maintain the graphite wafers 24 in engagement with the sink 8, a known type of spring clamp 26 comprising, for example, crossbars 28, rods 30, and springs 32 is used. The pressure provided by the clamp 26 is not critical. Pressures between 50 and 600 psi. have been found satisfactory. Preferably, an alumina ceramic insulating plate 34 is bonded to the sink 8 for engagement with the graphite wafers 24. This electrically insulates the thermoelectric generator from the heat sink.

In FIG. 2 is shown the lower or cold end of a thermoelectric generator 37. The hot end of the generator is similar to that of the generator 4 shown in FIG. 1. Like reference numbers designate like parts in the two embodiments. In this embodiment, a graphite wafer 24 is bonded to each end of the thermoelements N and P, that is, to the shoes 18 and 20, and a graphite wafer 40 is bonded to each insulating plate 34 bonded to the heat sink 8. The graphite wafers 40 may be diffusion bonded to the insulating plates 34 in the same manner as the graphite wafers 24 are bonded to the shoes 18 and 20. A spring clamp, not shown, similar to the one shown in FIG. 1 is used to provide pressures in the order of 50 to 600 psi. A graphite to graphite pressure contact is thus provided.

Although not shown, a graphite pressure contact may 'be used between the hot strap 11 of the thermoelectric generators 4 and 37 and the heat source 6, with the cold end of the thermoelectric generators rigidly bonded to the heat sink 8. Alternately, graphite pressure contacts may be used at both ends of the thermoelectric generators. Also, with graphite pressure contacts having only one graphite member, the graphite member may be bonded either to the thermoelement or to the heat source or sink. Further still, the graphite member need not be bonded at all, but held in place by the pressure clamp 26. Bonding of the graphite is generally preferred, however, since a bonded joint generally has lower thermal and electric resistances than a pressure joint.

The use of graphite as a surface of a pressure joint has several advantages. Graphite, for example, has a low modulus of elasticity, hence a high degree of compliance. Because of this, surface imperfections which tend to prevent intimate surface engagement between the graphite and its contacting member are readily flattened with relatively small pressures. The resulting intimate surface contact, as well as the relatively high thermal conductivity of graphite, provides a high thermal conductivity pressure joint or contact.

Graphite has a relatively low coefficient of friction even at high temperatures in vacuum, and is relatively chemically inactive with little tendency to react with or stick to other surfaces. Further, graphite surfaces are suitable for accepting certain vacuum stable solid lubricants, such as molybdenum disulfide, tungsten disulfide, and tungsten diselenide. The use of such lubricants further reduces the coefficient of friction of the graphite. Thus, even though under compression and in vacuum, the use of a graphite surface, with or without lubrication, provides a Sliding contact across which thermal stresses are not readily transmitted.

Typical temperature drops across graphite pressure contacts of the type described are in the range of 5 to 20 C. at a flux density of 20 to 25 watts/cm.

Thermoelectric devices of the type described have been operated with temperature differentials of 700 C. across 0.50 inch long silicon-germanium alloy thermoelements for 2500 hours in on-ofi? cyclic operation without cracking of the thermoelements.

I claim:

1. A thermoelectric device comprising at least two semiconductor thermoelements of opposite type conductivity and meansfor electrically connecting said thermoelements, each of said thermoelements having two ends, at least one of the ends of one of said thermoelements being bonded to a first contacting member of a material other than graphite, a first graphite member bonded to the other end of said one thermoelement, a second graphite member engaged with said first graphite member, a second contacting member of a material other than graphite bonded to said second graphite member, and means for resiliently pressing said graphite members together.

References Cited UNITED STATES PATENTS 885,430 4/1908 Bristol 136-239 2,289,152 7/1942 Telkes 136201 X Henderson et a1. 136-239 X FOREIGN PATENTS Germany.

ALLEN B. CURTIS, Primary Examiner.

US. Cl. X.R. 

1. A THERMOELECTRIC DEVICE COMPRISING AT LEAST TWO SEMICONDUCTOR THERMOELEMENTS OF OPPOSITE TYPE CONDUCTIVITY AND MEANS FOR ELECTRICALLY CONNECTING SAID THERMOELEMENTS, EACH OF SAID THERMOELEMENTS HAVING TWO ENDS, AT LEAST ONE OF THE ENDS OF ONE OF SAID THERMOELEMENTS BEING BONDED TO A FIRST CONTACTING MEMBER OF A MATERIAL OTHER THAN GRAPHITE, A FIRST GRAPHITE MEMBER BONDED TO THE OTHER END OF SAID ONE THERMOELEMENT, A SECOND GRAPHITE MEMBER ENGAGED WITH SAID FIRST GRAPHITE MEMBER, A SECOND CONTACTING MEMBER OF A MATERIAL OTHER THAN GRAPHITE BONDED TO SAID SECOND GRAPHIT MEMBER, AND MEANS FOR RESILIENTLY PRESSING SAID GRAPHITE MEMBERS TOGETHER. 